The present invention is directed to a method for generating hydrogen in a catalytic steam reformer.
In a conventional catalytic steam reforming process, a reformer feedstock is introduced into a desulfurization unit to remove sulfur and form a sulfur-depleted reformer feedstock. Sulfur in the reformer feedstock is removed to prevent poisoning of reforming catalyst. Additionally, in a hydrodesulfurization unit, at least a portion of the unsaturated hydrocarbons in the reformer feedstock are converted to saturated hydrocarbons.
The sulfur-depleted reformer feedstock is combined with a steam-containing gas (e.g. process steam) to form a sulfur-depleted feedstock mixture (mixed feed). Optionally, the sulfur-depleted feedstock mixture may be heated and then prereformed in a prereformer where a portion of the hydrocarbon is reformed by steam to form hydrogen and carbon monoxide.
The sulfur-depleted feedstock mixture or the effluent from the prereformer is introduced into a catalytic steam reformer as a reformer feed gas mixture. The reformer feed gas mixture may be optionally heated prior to being introduced into the catalytic steam reformer. In the catalytic steam reformer, hydrocarbon is reformed by steam over a reforming catalyst to form a reformed gas mixture. The reformed gas mixture generally comprises hydrogen, carbon monoxide, carbon dioxide, and unconverted hydrocarbon and steam.
Catalytic steam reformers have numerous reactor tubes containing steam reforming catalyst, called reformer tubes, generally arranged parallel to each other in the reformer. The reformer is a type of furnace where a fuel is combusted external to the reformer tubes to provide heat for the reforming reactions. Fuel and oxidant, typically air, is introduced through burners generating heat energy that is transferred to the reformer tubes by radiation and convection.
The reformer feed gas mixture is introduced into the reformer tubes where reforming reactions take place to form a reformed gas mixture. The reformed gas mixture from the reformer tubes is collected in a common header manifold. The reformed gas mixture is removed as effluent from the reformer. The reformed gas mixture is generally referred to as synthesis gas or syngas. The reformed gas mixture may be further processed in a shift reactor and/or separated in a hydrogen pressure swing adsorption system to form a hydrogen-containing product stream. Alternatively, the reformed gas mixture as syngas may be used for synthesizing other chemicals, for example methanol.
A large portion of the heat contained in this reformed gas mixture is recovered in a heat exchanger, referred to as a waste heat boiler to produce steam. The steam generated in the waste heat boiler may be used to form the steam-containing gas used for the reforming reaction and/or may be exported to another nearby process.
In addition to providing hydrogen to refineries, hydrogen production facilities are often required to provide a minimum amount of export steam under contract. Export steam is steam produced by the hydrogen production facility that is exported to another process.
There are occasions when the hydrogen production rate is decreased below the design capacity (i.e. turndown). The hydrogen production rate may be decreased due to normal variation in hydrogen demand. The hydrogen production rate may be decreased due to lack of demand at a refinery. The hydrogen production rate may be decreased due to lack of hydrogen pipeline demand.
A problem that occurs during hydrogen production turndown is that the amount of steam produced by the hydrogen production facility is also decreased. Part of this steam production decrease is due to the reduction of effluent available from the reformer and the associated heat from the effluent for the waste heat boiler. The amount of steam produced is directly coupled to the amount of hydrogen produced. This may lead to a situation where the demand on export steam cannot be met due to low hydrogen production.
It would be desirable to efficiently produce steam in a hydrogen production facility at various hydrogen production rates, particularly during turndown conditions.